Hard and abrasion resistant surfaces protecting cathode blocks of aluminium electrowinning

ABSTRACT

A component of an aluminium production cell, in particular a cathode or a cell lining of an electrolytic cell for the production of aluminium by the electrolysis of alumina in cryolite, having an aluminium-wettable refractory coating on a heat-stable baked carbon-containing body, is produced from a part-manufactured cell component which is a layered composite of two precursors. A precursor layer of the aluminium-wettable refractory coating contains at least one aluminium-wettable refractory material in particulate form, or a particulate micropyretic reaction mixture which when ignited reacts to form at least one aluminium-wettable refractory material, or a mixture thereof, and non-carbon fillers and binders. A non-baked or part-baked precursor of the heat-stable carbon-containing body comprises particulate carbon compacted with a heat-convertible binder which when subjected to heat treatment binds the particulate carbon into the heat-stable carbon-containing body of the fully-manufactured cell component. This layered composite is convertible to the fully-manufactured cell component by heat treatment to form the aluminium-wettable refractory coating and simultaneously bind and heat-stabilize the carbon-containing body.

This application is a 371 of PCT/IB96/00779, filed Aug. 6, 1996 and acontinuation in part of Ser. No. 08/511,907, filed Aug. 7, 1995, nowU.S. Pat. No. 5,728,466.

FIELD OF THE INVENTION

The invention relates to the application of refractory materials tocarbon cathode blocks of cells for the electrowinning of aluminium byelectrolysis of alumina dissolved in a cryolite-based moltenelectrolyte. The invention also relates to the cathode blocks and tosuch cells incorporating them.

BACKGROUND OF THE INVENTION

Aluminium is produced conventionally by the Hall-H{acute over (e)}roultprocess, by the electrolysis of alumina dissolved in cryolite-basedmolten electrolytes at temperatures up to around 950° C. A Hall-H{acuteover (e)}roult reduction cell typically has a steel shell provided withan insulating lining of refractory material, which in turn has a liningof carbon which contacts the molten constituents. Conductor barsconnected to the negative pole of a direct current source are embeddedin the carbon cathode substrate forming the cell bottom floor. Thecathode substrate is usually a carbon lining made of prebakedanthracite-graphite or all graphite carbon blocks, joined with a rammingmixture of anthracite, coke, and coal tar.

In Hall-H{acute over (e)}roult cells, a molten aluminium pool acts asthe cathode. The carbon lining or cathode material has a useful life ofthree to eight years, or even less under adverse conditions. Thedeterioration of the cathode bottom is due to erosion and penetration ofelectrolyte and liquid aluminium as well as intercalation of sodium,which causes swelling and deformation of the cathode carbon blocks andramming mix. In addition, the penetration of sodium species and otheringredients of cryolite or air leads to the formation of toxic compoundsincluding cyanides.

Difficulties in operation also arise from the accumulation ofundissolved alumina sludge on the surface of the carbon cathode beneaththe aluminium pool which forms insulating regions on the cell bottom.Penetration of cryolite and aluminium through the carbon body and thedeformation of the cathode carbon blocks also cause displacement of suchcathode blocks. Due to cracks in the cathode blocks, aluminium reachesthe steel cathode conductor bars causing corrosion thereof leading todeterioration of the electrical contact, non uniformity in currentdistribution and an excessive iron content in the aluminium metalproduced.

A major drawback of carbon as cathode material is that it is not wettedby aluminium. This necessitates maintaining a deep pool of aluminium(100-250 mm thick) in order to ensure a certain protection of the carbonblocks and an effective contact over the cathode surface. Butelectromagnetic forces create waves in the molten aluminium and, toavoid short-circuiting with the anode, the anode-to-cathode distance(ACD) must be kept at a safe minimum value, usually 40 to 60 mm. Forconventional cells, there is a minimum ACD below which the currentefficiency drops drastically, due to short-circuiting between thealuminium pool and the anode or to oxidation of the aluminium produced.The electrical resistance of the electrolyte in the inter-electrode gapcauses a voltage drop from 1.8 to 2.7 volts, which represents from 40 to60 percent of the total voltage drop, and is the largest singlecomponent of the voltage drop in a given cell.

To reduce the ACD and associated voltage drop, extensive research hasbeen carried out with Refractory Hard Metals or Refractory HardMaterials (RHM) such as TiB₂ as cathode materials. TiB₂ and other RHM'sare practically insoluble in aluminium, have a low electricalresistance, and are wetted by aluminium. This should allow aluminium tobe electrolytically deposited directly on an RHM cathode surface, andshould avoid the necessity for a deep aluminium pool. Because titaniumdiboride and similar Refractory Hard Metals are wettable by aluminium,resistant to the corrosive environment of an aluminium production cell,and are good electrical conductors, numerous cell designs utilizingRefractory Hard Metal have been proposed, which would present manyadvantages, notably including the saving of energy by reducing the ACD.

The use of titanium diboride and other RHM current-conducting elementsin electrolytic aluminium production cells is described in U.S. Pat.Nos. 2,915,442, 3,028,324, 3,215,615, 3,314,876, 3,330,756, 3,156,639,3,274,093 and 3,400,061. Despite extensive efforts and the potentialadvantages of having surfaces of titanium diboride at the cell cathodebottom, such propositions have not been commercially adopted by thealuminium industry.

The non-acceptance of tiles and other methods of applying layers of TiB₂and other RHM materials on the surface of aluminium production cells isdue to their lack of stability in the operating conditions, in additionto their cost. The failure of these materials is associated withpenetration of the electrolyte when not perfectly wetted by aluminium,and attack by aluminium because of impurities in the RHM structure. InRHM pieces such as tiles, oxygen impurities tend to segregate alonggrain boundaries leading to rapid attack by aluminium metal and/or bycryolite. To combat disintegration, it has been proposed to use highlypure TiB₂ powder to make materials containing less than 50 ppm oxygen.Such fabrication further increases the cost of the already-expensivematerials. No cell utilizing TiB₂ tiles as cathode is known to haveoperated for long periods without loss of adhesion of the tiles, ortheir disintegration. Other reasons for failure of RHM tiles have beenthe lack of mechanical strength and resistance to thermal shock.

Various types of TiB₂ or RHM layers applied to carbon substrates havefailed due to poor adherence and to differences in thermal expansioncoefficients between the titanium diboride material and the carboncathode block.

PCT patent application WO93/25731 describes a carbon-containingcomponent of a cell for the production of aluminium by the electrolysisof alumina dissolved in a cryolite-based molten electrolyte, which cellcomponent is protected from attack by liquid and/or gaseous componentsof the electrolyte or products produced during cell operation by acoating of particulate refractory hard metal boride and a colloidbonding applied from a slurry of the boride in a colloidal carrier whichcomprises at least one of colloidal alumina, silica, yttria, ceria,thoria, zirconia, magnesia, lithia, monoaluminium phosphate or ceriumacetate.

The method of applying the refractory coating of WO93/25731 involvedapplying to the surface of the component a slurry of particulaterefractory boride in the colloidal carrier, followed by drying and aheat treatment to consolidate the boride coating without any reactiontaking place, this heat treatment preferably being in air or otheroxidising atmospheres or alternatively in inert or reducing atmospheres.A heat treatment in air at about 80-200° C., for half an hour to severalhours was found to be sufficient.

WO 95/13407 discloses manufacturing an aluminium electrowinning cellcomponent having a refractory coating on a carbon body starting from arefractory coating precursor and a green carbon body that are bakedtogether, to simultaneously produce the coating and bake the carbonbody.

SUMMARY OF THE INVENTION

The invention aims to further improve the refractory coatings describedin WO93/25731 as surface coatings on carbonaceous substrates, forprotecting the substrates from the corrosive attacks of liquids andgases when used as cell components for aluminium production cells,especially for use as cathodes.

An object of the invention is to provide improved hard coatingscontaining borides that have exceptional adherence to thecarbon-containing substrates, provide the required protection to thecell components and have improved mechanical, physical, chemical, andelectrochemical characteristics.

Another object of the invention is to provide an improved method ofapplying refractory borides to carbon-containing cell components to forma hard coating wherein a part of the boride is reacted by heat treatmentbefore use of the cell component to improve its protection.

According to the invention, an improved hard surface is provided on acarbon body to be used as cathode block in cells for the electrowinningof aluminium by adding to the surface of the block one or more layerscontaining particulate refractory hard metal boride and bonding materialwhich when heated reacts with the refractory hard metal boride andcarbon from the carbon body or from a carbon-containing atmosphere, toform the hard surface.

Such improved surfaces are very hard; resistant to sodium penetrationwhereby sodium penetration is controlled; abrasion resistant henceresistant to erosion, corrosion resistant; preferably wettable by moltenaluminium; and resistant to attack by molten aluminium.

The surface layer usually contains 30-97 wt % of refractory hard metalboride, 3-50 wt % bonding material and 0-50 wt % of non-reactivefillers.

The carbon body to be used as cathode block may initially be a green(i.e. non-baked or part-baked) body comprising particulate carboncompacted with a heat-convertible binder which, when subjected to theheat treatment under the non-oxidising atmosphere, binds the carbon intoa final heat stable carbon body during the reaction forming the hardsurface. This heat-convertible binder can be a carbon-based materialsuch as pitch, or may be non-carbonaceous. During the heat treatment,carbon from the green carbon body or from a carbon-containingatmosphere, reacts with the refractory hard metal and/or the bondingmaterial.

The coating's bonding material and filler, if present, are generallycarbon-free, which avoids the known problems of refractory borideembedded in carbon binders which are unstable in contact with moltenaluminium.

The preferred non-carbon bonding materials are colloids selected fromcolloidal alumina, silica, yttria, ceria, thoria, zirconia, magnesia,lithia, monoaluminium phosphate or cerium acetate.

In the reaction which takes place under the non-oxidising atmosphere,the colloid and particulate refractory boride react to form at least onestable compound between at least two elements selected from: at leastone element derived from the colloid; boron derived from the refractorymetal boride; refractory metal derived from the refractory metal boride;carbon and oxygen.

Preferably, the colloid is colloidal alumina and the hard surfacecomprises at least one compound formed between aluminium derived fromthe colloid and at least one of: the refractory hard metal; boron; andcarbon. Such compounds; include Ti₃AlC, AlB₁₀, Al₄B₂O₉, Al₁₈B₄O₃₃ andAl(BO₃)O₆. Other possible compounds include Al₃Ti and AlB₂.

As known from WO 93/25731, the colloid may be derived from colloidprecursors and reagents which are solutions of at least one salt such aschlorides, sulfates, nitrates, chlorates, perchlorates or metal organiccompounds such as alkoxides, formates, acetates of aluminium, silicon,yttrium, cerium, thorium zirconium, magnesium and lithium. These colloidprecursors or colloid reagents can contain a chelating agent such asacetyl acetone or ethylacetoacetate. The aforesaid solutions of metalorganic compounds, principally metal alkoxides, can be of the generalformula M(OR)_(z) where M is a metal or complex cation, R is an alkylchain and z is a number, preferably from 1 to 12.

In particular, an aluminium-wettable, refractory, electricallyconductive, adherent boride coating has been developed containingcompounds such as Ti₃AlC and AlB₁₀, which coatings are applied to thesurface of the cathode block made of carbonaceous material before use inan aluminium electrowinning cell, to enhance protection of thecarbonaceous material from the attack of sodium and air which producesdeformation of the cathode blocks and formation of dangerous nitrogencompounds such as cyanides, and attack by molten aluminium.

By improving protection of the carbonaceous cell components from attackby NaF or other aggressive ingredients of the electrolyte and moltenaluminium, the cell efficiency is further improved. Because NaF in theelectrolyte no longer reacts with the carbon cell bottom, the cellfunctions with a defined bath ratio without a need to replenish theelectrolyte with NaF.

The aluminum-wettable refractory boride coating containing compoundssuch as Ti₃AlC or AlB₁₀, will also permit the elimination of the thickaluminium pool required to partially protect the carbon cathode,enabling the cell to operate with a drained cathode.

The protective effect of the coatings according to the invention is suchas to enable the use of relatively inexpensive carbon-containingmaterials for the cathode blocks. For instance, cheaper grades ofgraphite can be used instead of the more expensive anthracite forms ofcarbon, while providing improved resistance against the corrosiveconditions in the cell environment.

The hard and abrasion-resistant coatings containing said furthercompounds have the following attributes: excellent wettability by moltenaluminium, excellent adherence to the carbon-containing substrates,inertness to attack by molten aluminium and cryolite, particularly withlow levels of titanium and boron in the molten aluminium, sufficient toinhibit dissolution of the coating, low cost, environmentally safe,ability to absorb thermal and mechanical shocks without delaminationfrom the anthracite-based carbon or other carbonaceous substrates,durability in the environment of an aluminium production cell, and easeof application and processing. The preferred coatings furthermore have acontrolled microporosity and degree of penetration in the porouscarbonaceous substrate, by having an adequate distribution of theparticle sizes of the preformed refractory boride.

Compared to the corresponding boride coatings of WO/93/25731 withoutsaid further compounds, the coatings of this invention with furthercompounds such as Ti₃AlC and AlB₁₀, have the following advantages:greater hardness and resistance to mechanical wear, for example due tomovements in the aluminium pool; better adhesion to the carbonsubstrate; and essentially complete inertness to reaction with moltenaluminium or other molten cell components.

When these refractory boride coatings containing said further compoundsare applied for instance to graphite or anthracite-based carbon used ascathode blocks, the hard surface protect the substrate against theingress of cryolite and sodium and is in turn protected by theprotective film of aluminium on the coating itself.

The hard surfaces are also useful in cells where the temperature ofoperation is low as in the Low Temperature electrolysis process for theproduction of aluminium (see for example U.S. Pat. No. 4,681,671 and PCTapplication PCT/EP92/02666).

The particulate refractory boride is selected from borides of titanium,chromium, vanadium, zirconium, hafnium, niobium, tantalum, molybdenumand cerium. The preferred particulate refractory boride is titaniumdiboride. These borides may be used alone or in combination withrefractory hard metal carbides.

As known from WO 93/25731, when choosing powder additives the particlesize selection is of importance. It is preferable to choose particlesize below 100 micrometers and to choose particle sizes which are variedsuch that the packing of particles is optimized. For example it ispreferable to choose particle sizes extending over a range where thesmallest particles are at least two times and preferably at least threetimes smaller than the large ones. Generally, the ratio of the particlesizes will be in the range from 2:1 to 15:1, usually from about 3:1 to10:1, for instance a ratio of about 3:1 with large particles in therange 15 to 30 micrometers and small particles in the range 5 to 10micrometers, or a ratio of about 10:1 with large particles in the rangefrom 30 to 50 micrometers and small particles in the range from 3 to 5micrometers. Usually, the particulate metal boride has particles withsizes in the range from about 3 micrometers to about 50 micrometers.

To apply the particulate refractory boride, a slurry is formed with atleast one of these colloids in a liquid carrier. The slurry usuallycontains 5-100 g of the particulate refractory boride per 10 ml ofcolloid and the colloid has a dry colloid content corresponding to up to50 weight % of the colloid plus liquid carrier, preferably from 10 to 20weight %.

The colloid is contained in a liquid such as water which may furthercontain at least one compound selected from compounds of lithium,aluminium, cerium, sodium and potassium, for instance at least onecompound of lithium and at least one compound of aluminium, see PCTpatent application WO 94/21573, the contents whereof are incorporatedherein by way of reference.

Method of Production

Another aspect of the invention is a method of protectingcarbon-containing cathodes from the attack of cryolite, molten aluminiumand sodium, and improving their hardness and erosion resistance,comprising applying to the carbon body for forming the cathode block alayer of particulate refractory hard metal boride with theaforementioned bonding materials, and heating the body under anon-oxidising atmosphere to cause the bonding material to react with therefractory hard metal boride and carbon from the carbon-containing bodyor from a carbon-containing atmosphere, to form the surface layer ofimproved hardness and abrasion resistance.

The method preferably involves applying to the surface of the body aslurry of particulate refractory boride in a liquid carrier includingthe bonding material, followed by drying, and by the reactive heattreatment under a non-oxidising atmosphere before or after the body isinstalled as cathode in an aluminium production cell, but before use ofthe cell.

The method of application of the slurry involves painting (by brush orroller), dipping, spraying, or pouring the slurry onto the substrate andallowing for drying before another layer is added. The coating need notentirely dry before the application of the next layer, but it ispreferred to heat the coating with a suitable heat source so as tocompletely dry it and improve densification of the coating. Heating thentakes place in an inert or a reducing atmosphere, preferably with thecolloid-applied coating under a bed of carbon or under acarbon-containing atmosphere such as CO/CO₂ possibly mixed withnitrogen.

Heating under an inert or a reducing atmosphere, in particular acarbon-containing inert atmosphere, may take place at about 850°C.-1300° C., usually 900° C.-1200° C., for at least 10 hours.

Excellent results are obtained at about 950° C. for about 24 hours.

In view of the great stability of refractory borides such as TiB₂, it isextremely surprising to find that by sustained heating under an inert ora reducing atmosphere—particularly a carbon-based reducingatmosphere—these borides in the presence of the colloid react to formstable compounds with elements derived from the colloid and with carbon.

As these reactions are carried out before the boride-based coating isexposed to molten aluminium and other molten components of the cellenvironment, the resistance of the coating to the molten cell componentsduring use of the cell is enhanced.

The bonding material preferably comprises at least one colloid selectedfrom colloidal alumina, silica, yttria, ceria, thoria, zirconia,magnesia, lithia, monoaluminium phosphate or cerium acetate.

The colloid and particulate refractory boride react to form at least onestable compound between at least one element derived from the colloidand at least one of: boron derived from the refractory metal boride;refractory metal derived from the refractory metal boride; and carbonand oxygen. In particular, the colloid is colloidal alumina which reactsto form at least one compound between aluminium derived from the colloidand at least one of: the refractory hard metal; boron; and carbon.

The slurry is usually applied in several layers, each layer beingallowed to dry at least partially in the ambient air or assisted byheating before applying the next layer, followed by a final heattreatment to dry the slurry after application of the last layer.

The slurry usually comprises 5-100 g of the particulate refractoryboride per 10 ml of colloid, and the colloid has a dry colloid contentcorresponding to up to 50 weight % of the colloid plus liquid carrier,preferably from 10 to 20 weight %. The colloid may be contained in aliquid carrier which further contains at least one compound selectedfrom compounds of lithium, aluminium, cerium, sodium and potassium.

The substrate may be treated by sand blasting or pickled with acids orfluxes such as cryolite or other combinations of fluorides and chloridesprior to the application of the coating. Similarly the substrate may becleaned with an organic solvent such as acetone to remove oily productsand other debris prior to the application of the coating. Thesetreatments will enhance the bonding of the coatings to thecarbon-containing substrate.

Generally, before or after application of the hard surface and beforeuse, the body can be painted, sprayed, dipped or infiltrated withreagents and precursors, gels and/or colloids. For instance, beforeapplying the slurry of particulate refractory boride in the colloidalcarrier the carbonaceous component can be impregnated with e.g. acompound of lithium to improve the resistance to penetration by sodium,as described in PCT patent application WO 94/20650 the contents whereofare incorporated herein by way of reference.

To assist rapid wetting of the cathode blocks by molten aluminium, thealuminium wettable surface layer may be exposed to molten aluminium, forexample in the presence of a flux assisting penetration of aluminiuminto the refractory material, the flux for example comprising afluoride, a chloride or a borate of lithium or sodium, or mixturesthereof. Such treatment favors aluminization of the refractory coatingby the penetration therein of aluminium.

The cathode block may be coated outside the aluminium production celland the coated block then inserted into the cell. Alternatively, thecathode block is already assembled in a cell bottom and coating takesplace in the cell prior to operation. Thus, the block is part of a cellbottom forming by an exposed area of carbonaceous material. In thiscase, the slurry is preferably applied to the cell bottom in severallayers with drying of each successive layer, drying by means of a mobileheat source and heat treatment under an inert or a reducing atmosphereto produce the desired reactions.

As mentioned the carbon body may be a green body i.e. is a non-baked orpart-baked body comprising particulate carbon compacted with aheat-convertible binder which when subjected to the heat treatment bindsthe carbon into a final heat stable carbon body and reacts with thecoating components to form the aluminium-resistant coating.

Cell Components

The invention concerns principally carbon cathode blocks and alsoconcerns other cell components of aluminium production cells, inparticular components which in use are exposed to corrosive or oxidisinggas released in operation of the cell or present in the cell operatingconditions, which components are protected from corrosion or oxidationby the aluminium resistant surface layer as set out above.

According to the invention, there is provided a component of a cell forthe electrowinning of aluminium, comprising a carbon body having a hardand preferably aluminium wettable surface layer obtained by heating anapplied layer of particulate refractory hard metal boride with at leastone bonding material which when heated reacts with the refractory hardmetal boride and carbon from the carbon-containing body or from acarbon-containing atmosphere.

This cell component can incorporate all of the features of theabove-described carbon cathode block with its hard surface layer, inparticular it is protected from attack by liquid and/or gaseouscomponents of the electrolyte in the form of elements, ions or compound,by a coating of pre-formed particulate refractory hard metal boride andother compounds formed by reaction with the bonding material.

The component may be a current-carrying component for example a cathode,a cathode current feeder, or a bipolar electrode coated on its cathodeface.

The component may also be a non current-carrying component for example acell sidewall.

The slurry-applied refractory boride coatings containing said otherreaction-formed compounds may have a thickness from about 150micrometers to about 1500 micrometers, usually from about 200 to about500 micrometers, depending on the number of applied layers, the particlesize of the preformed boride, and the porosity of the carbon.Advantageously, by using graded boride particles including fineparticles, the smaller boride particles penetrate into the pores of thecarbon component and firmly anchor the coating. Typically, the boridemay impregnate the carbon to a depth of about 50-200 micrometers, andthe aforesaid compounds may also be formed in this zone. The colloidimpregnates the carbon component so the dried colloid is dispersedthrough the carbon component.

The invention concerns in general the protection of components ofelectrochemical cells for the electrowinning of aluminium by theelectrolysis of alumina dissolved in a cryolite-based moltenelectrolyte, which components in use are exposed to a corrosiveatmosphere, or to a molten cryolite, and/or to a product of electrolysisin the cell, in particular to molten aluminium. Such components arecoated with a hard protective surface coating which improves theresistance of the components to oxidation or corrosion and which mayalso enhance the electrical conductivity and/or electrochemicalactivity. The protective coating is preferably applied from a colloidalslurry containing particulate preformed refractory boride and dried.When the component is heated to a sufficient elevated temperature underan inert or a reducing atmosphere, prior to or upon insertion in thecell but before use of the cell, a protective coating is formed withreaction between compounds of the colloid, boron from the boride, therefractory metal from the boride and with carbon.

The invention also concerns a component of an aluminium production cellwhich in use is subjected to exposure to molten cryolite and/or tomolten aluminium or corrosive fumes or gases, the component comprising asubstrate of a carbonaceous material, coated with a refractory boride,of at least one of titanium, chromium, vanadium, zirconium, hafnium,niobium, tantalum, molybdenum and cerium or mixtures thereof, finelymixed with a refractory compound of at least one alumina, silica,yttria, ceria, thoria, zirconia, magnesia and lithia from a driedcolloid and at least one stable compound formed between at least oneelement derived from the colloid (in particular aluminium from colloidalalumina); boron derived from the refractory metal boride; refractorymetal derived from the refractory metal boride; carbon; and oxygen.

The component is usually made of carbonaceous material selected frompetroleum coke, metallurgical coke, anthracite, graphite, amorphouscarbon, fullerene, low density carbon or mixtures thereof. Compositematerials based on one or more of these forms of carbon with othermaterials may also be employed.

It is possible for the component to have a substrate of low-densitycarbon protected by the refractory boride, for example if the componentis exposed to oxidising gas released in operation of the cell, or alsowhen the substrate is part of a cell bottom. Low density carbon embracesvarious types of relatively inexpensive forms of carbon which arerelatively porous and very conductive, but hitherto could not be usedsuccessfully in the environment of aluminium production cells on accountof the fact that they were subject to excessive corrosion or oxidation.Now it is possible by coating these low density carbons according to theinvention, to make use of them in these cells instead of the moreexpensive high density anthracite and graphite, taking advantage oftheir excellent conductivity and low cost.

The substrate usually consists of carbonaceous blocks that can be fittedtogether to form a cell bottom of an aluminium production cell, or maybe packed carbonaceous particulate material forming a cell bottom, whichacts to carry current to the cathodic pool if there is one, or to a thinlayer of aluminium through the refractory boride coating in drainedcells.

The component advantageously forms part of a cathode through which theelectrolysis current flows, the hard coating containing said furthercompounds forming a cathodic surface in contact with thecathodically-produced aluminium. For example, it is part of a drainedcathode, the refractory boride coating forming the cathodic surface onwhich the aluminium is deposited cathodically, and the component beingarranged usually upright or at a slope for the aluminium to drain fromthe cathodic surface.

Electrolytic Cells and Operation

The invention also relates to an aluminium production cell comprising acoated component as discussed above as well as a method of producingaluminium using such cells and methods of assembling and/or operatingthe cells.

Such cells may comprise a component which in operation of the cell isexposed to molten cryolite or aluminium, said component comprising asubstrate of carbonaceous material and a coating of refractory boride,applied from a colloidal slurry and reacted as discussed above, whereinthe product aluminium is in contact with the hard surface on thecomponent, which may be a cathode or forms part of a cathodic cellbottom.

It should be noted that aluminium-containing compounds such as Ti₃AlC,or AlB₁₀ in the coating are produced by reaction with aluminium from thecolloid such as colloidal alumina. This reaction takes place during theheat treatment under an inert or reducing atmosphere, prior to use ofthe cell. The reaction does not take place in contact with the productaluminium. These pre-formed compounds make the coating essentially inertto the product aluminium.

The invention also concerns an aluminium production cell having acomponent which in operation of the cell is exposed to corrosive oroxidising gas released in operation of the cell or present in the celloperating conditions, or exposed to molten cryolite, said componentcomprising a substrate of carbonaceous material, and a coating ofrefractory boride deposited in particular from a colloidal slurry andreacted, as discussed above.

A method of operating the cells comprises:

producing a cell component which comprises a substrate of carbonaceousmaterial and a hard surface of refractory boride by applying to thecarbon substrate a layer of particulate refractory hard metal boridewith bonding material as discussed above, and heating the body under anon-oxidising atmosphere to cause the refractory hard metal boride toreact to form an aluminium resistant surface layer, in particular by themethods as described above;

placing the coated component in the cell so the hard surface will becontacted by the cathodically produced aluminium, and/or the moltenelectrolyte, and/or the anodically-released gases; and

operating the cell with the hard surface protecting the substrate fromattack by the cathodically-produced aluminium and by the moltenelectrolyte (and possibly by the anodically-released gases with which itis in contact).

Operation of the cell may be under standard conditions encountered inHall-H{acute over (e)}roult cells, or in a low temperature process, withthe molten halide electrolyte containing dissolved alumina at atemperature below 900° C., usually at a temperature from 680° C. to 880°C. The low temperature electrolyte may be a fluoride melt or a mixedfluoride-chloride melt.

This low temperature process is operated at low current densities onaccount of the low alumina solubility. This necessitates the use oflarge anodes and corresponding large cathodes, exposing large areas ofthese materials to the corrosive conditions in the cell, such largeexposed areas being well protected by the refractory coatings accordingto the invention which are just as advantageous at these lowertemperatures.

Generally, the improved coatings of this invention can be applied in asimilar manner to those described and illustrated in WO 93/25731 butwith the heat treatment under an inert or a reducing atmosphere to reactthe coatings as described above, and as further illustrated in thefollowing Examples.

EXAMPLE I

A slurry was prepared from a dispersion of 25 g TiB₂, 99.5% pure, −325mesh (<42 micrometer), in 10 ml of colloidal alumina containing about 20weight % of solid alumina. Coatings with a thickness of 150±50 to 500±50micrometer were applied to the faces of carbon blocks. Each layer ofslurry was allowed to dry for about one hour (generally from severalminutes up to a few hours) before applying the next, followed by adrying by baking in an oven at 100-150° C. for 30 minutes to 1 hour.

The above procedure was repeated varying the amount of TiB₂ in theslurry from 5 to 15 g and varying the amount of colloidal alumina from10 ml to 40 ml. Coatings were applied as before. Drying in air took 10to 60 minutes depending on the dilution of the slurry and the thicknessof the coatings. In all cases, an adherent layer of TiB₂ was obtained.

The coated carbon blocks were then placed under a layer of powderedcarbon and heated at 900° C.-1000° C. for 18-36 hours, typically at 950°C. for 24 hours. This heating takes place in a furnace under air, butthe presence of the carbon powder on the coating ensures that thecoating is effectively exposed to a reducing atmosphere of CO/CO₂containing nitrogen.

After cooling, analysis of the coating revealed the presence of a TiB₂layer adhering firmly to the carbon substrate, containing substantialamounts of Ti₃AlC and AlB₁₀ and traces of Al₄B₂O₉, Al₁₈B₄O₃₃ andAl(BO₃)O₆ showing that the titanium and the boron from the TiB₂ hadreacted with aluminium from the colloidal alumina, with carbon from thereducing CO/CO₂ atmosphere, the covering layer and/or from thesubstrate, and with oxygen from the reducing CO/CO₂ atmosphere.

When tested as cathode in a laboratory aluminium production cell, thesample showed good wettability with molten aluminium and no sign ofdeterioration. The aluminium was found to penetrate the coating andremain there.

EXAMPLE II

An anthracite-based cathode sample was coated with an adherent layercontaining TiB₂ as follows.

A layer of particulate TiB₂, 99.5% pure, was applied to an anthracitecathode sample in three coats using a solution of 25 g TiB₂ −325 mesh(<42 micrometer) in 10 ml of colloidal alumina containing about 20% ofthe colloid. Each coating had a thickness of 150±50 micrometer, and wasdried for about an hour before applying the next coating. The sample wasthen finally dried in air at about 120° C. for about ½ hour to 1 hour.

The coated carbon blocks were then placed under a layer of powderedcarbon and heated at 900° C.-1000° C. for 18-36 hours, typically at 950°C. for 24 hours. This heating takes place in a furnace under air, butthe presence of the carbon powder on the coating ensures that thecoating is effectively exposed to a reducing atmosphere of CO/CO₂containing nitrogen. After cooling, analysis of the coating revealed thepresence of a TiB₂ layer adhering firmly to the anthracite substrate,containing substantial amounts of Ti₃AlC and AlB₁₀ and traces ofAl₄B₂O₉, Al₁₈B₄O₃₃ and Al(BO₃)O₆, as in Example I.

EXAMPLE III

Specimens were machined from a commercial green carbon cathode block andwere coated with a layer of TiB₂ approximately 1.5-2 mm thick followingthe procedure of Example I.

The coated green carbon blocks were then packed into a graphitecrucible, covered with coke grains, placed in a furnace and baked at1200° C. for 12 hours.

The coatings were found be exceptionally hard and abrasion resistant.The measured hardness was from 63-80° Sh (average 77° Sh), compared to37-38° Sh for uncoated specimens.

The abrasion resistance, expressed as % weight loss measured by agrinding method, was 0.01-0.7% (average 0.04%) compared to 1.1-1.3% foruncoated samples.

EXAMPLE IV

Specimens were machined from a commercial pre-baked carbon cathode blockand were coated with a layer of TiB₂ approximately 1,5-2 mm thickfollowing the procedure of Example I.

The carbon blocks were then packed into a graphite crucible, coveredwith coke grains, placed in a furnace and re-baked at 950° C. or 1000°C. for 12 hours.

The measured hardness (average and extreme values) and abrasionresistance of the surfaces were as shown in Table I.

TABLE I Rebaking Abrasion resistance Temperature Hardness °Sh weightloss, % 950° C. 48 0.02 (44 ÷ 52) 100° C. 62 0.0 (57 ÷ 64)

EXAMPLE V

Example III was repeated except that, after allowing each applied coatedlayer of TiB₂ to dry for about 1 hour before applying the next layer,the coated green carbon blocks, without prebaking, were furnace bakedunder coke at 1200° C. for 12 hours to simultaneously bake and react thecoating, and bake the carbon blocks. As before, the finished surface hadexceptional hardness and abrasion resistance.

What is claimed is:
 1. A method for producing an improved carbon body tobe used as cathode block in cells for the electrowinning of aluminum, towhich a hard surface is provided by applying to the surface of thecarbon body one or more layers containing particulate refractory hardmetal boride and a carbon-free bonding material which when the carbonbody is heated reacts with the refractory hard metal boride and carbonfrom the surface of the carbon body or from a carbon-containingatmosphere, comprising applying to the carbon body one or more layers ofparticulate refractory hard metal boride with the bonding material, andheating the body under a non-oxidizing atmosphere to cause the bondingmaterial to react with the refractory hard metal boride and carbon fromthe carbon-containing body or from a carbon-containing atmosphere, toform the hard surface layer.
 2. The method of claim 1, comprisingapplying to the surface of the carbon body a slurry of particulaterefractory boride in a liquid carrier including the bonding material,followed by drying, and by reactive heat treatment under a non-oxidisingatmosphere before or after the body is installed as cathode block in analuminium production cell, but before use of the cell.
 3. The method ofclaim 1, wherein the heat treatment is carried out under a bed of carbonpowder to form a CO/CO₂ atmosphere.
 4. The method of claim 1, whereinthe heat treatment is carried out at 850° C.-1300° C. for at least 10hours.
 5. The method of claim 2, wherein the bonding material comprisesat least one colloid selected from colloidal alumina, silica, yttria,ceria, thoria, zirconia, magnesia, lithia, monoaluminium phosphate orcerium acetate.
 6. The method of claim 5, wherein the colloid andparticulate refractory boride react to form at least one stable compoundbetween at least one element derived from the colloid with at least oneof: boron derived from the refractory metal boride; refractory metalderived from the refractory metal boride; carbon; and oxygen.
 7. Themethod of claim 5, wherein the colloid is colloidal alumina and reactsto form at least one compound between aluminium derived from the colloidand at least one of: the refractory hard metal; boron; and carbon. 8.The method of claim 2, wherein the slurry is applied in several layers,each layer being allowed to dry at least partially in the ambient air orassisted by heating before applying the next layer, followed by a heattreatment to dry the slurry after application of the last layer, and byreactive heat treatment.
 9. The method of claim 1, wherein theparticulate refractory boride is selected from borides of titanium,chromium, vanadium, zirconium, hafnium, niobium, tantalum, molybdenumand cerium.
 10. The method of claim 9, wherein the particulaterefractory boride is titanium diboride.
 11. The method of claim 9,wherein the particulate refractory boride has a particle size below 100micrometers.
 12. The method of claim 11, wherein the particulaterefractory boride comprises particles of different sizes to optimizepacking of the particles, with a particle size ratio of at least 2:1.13. The method of claim 12, wherein the particle size ratio of theparticulate refractory boride is in the range 3:1 to 10:1.
 14. Themethod of claim 12, wherein the particulate refractory boride hasparticles with sizes in the range from about 3 micrometers to about 50micrometers.
 15. The method of claim 5, wherein the slurry comprises5-100 g of the particulate refractory boride per 10 ml of colloid. 16.The method of claim 15, wherein the colloid has a dry colloid contentcorresponding to up to 50 weight % of the colloid plus liquid carrier.17. The method of claim 15, wherein the colloid has a dry colloidcontent corresponding to from 10 to 20 weight % of the colloid plusliquid carrier.
 18. The method of claim 5, wherein the colloid iscontained in a liquid carrier which further contains at least onecompound selected from compounds of lithium, aluminium, cerium, sodiumand potassium.
 19. The method of claim 5, wherein the liquid carriercontains at least one compound of lithium and at least one compound ofaluminium.
 20. The method of claim 1, wherein the applied layer(s)contain sufficient refractory hard metal boride to render the resultinghard surface wettable by molten aluminium.
 21. The method of claim 20,wherein after drying and heat treatment the aluminium wettable hardsurface layer is aluminized by contact with molten aluminium.
 22. Themethod of claim 2, wherein the refractory boride is applied by dippingthe body in the slurry, painting, spraying or combinations of suchapplication techniques, in single or multi-layer coatings.
 23. Themethod of claim 1, wherein the carbon body is painted, sprayed, dippedor infiltrated with reagents and precursors, gels and/or colloids beforeand/or after the reactive heat treatment to produce the hard surfacelayer.
 24. The method of claim 1, wherein the carbon body is a greenbody comprising particulate carbon compacted with a heat-convertiblebinder which when subjected to the heat treatment binds the carbon intoa final heat stable carbon body, and wherein carbon from the carbon bodyreacts with at least one of the bonding material and refractory hardmetal boride.
 25. A method of producing a hard, abrasion-resistantaluminium-wettable surface layer on a carbon body to be used as cathodeblock in cells for the electrowinning of aluminium, comprising applyinga layer of particulate refractory metal boride with bonding material, toa green body comprising particulate carbon compacted with aheat-convertible binder which when subjected to the heat treatment bindsthe carbon into a final heat stable carbon body, and subjecting the bodyto reactive heat treatment in a non-oxidising atmosphere at 850°C.-1300° C. for at least 10 hours during which carbon from the carbonbody or contained in the non-oxidising atmosphere reacts with thebonding material and the refractory hard metal boride.